1. Field of the Invention
The invention relates generally to control techniques for fiberoptic gyros, and especially to apparatus and methods for time division multiplexing in controlling a plurality of fiberoptic gyros.
2. Description of the Related Art
A fiberoptic interferometer used for rotation sensing and measurement generally comprises a coherent source of light, a closed optical path, means for coupling light from the source into and out of the closed path, and means for detecting and processing the optical interference signal coming from the closed path.
There are two types of disturbances in an optical path that can give rise to phase shifts in light waves traveling in opposite directions around a closed optical path: reciprocal and nonreciprocal. A reciprocal disturbance is one that affects either light wave in a similar manner despite the fact that the two waves are traveling in different directions and may be subjected to the disturbance at different times. A nonreciprocal disturbance affects the two waves differently, either because it occurs over a time interval comparable to the time it takes a wave to travel around the closed path, or because the effect it has on a wave depends on the direction of propagation of the wave around the closed path.
The Sagnac effect, a relativistic physical phenomenon, is a nonreciprocal effect in which the rotation of a closed optical path causes light waves propagating in opposite directions along the path to take different amounts of time to complete a transit of the closed path. The difference in transit time results in a phase difference between the two light waves proportional to rotation rate. When the beams are recombined on a photodetector, they give rise to an interference pattern which is a function of the nonreciprocal phase shift. Measurement of the phase difference is a measure of the rate of rotation of the optical path.
If .phi..sub.s denotes the Sagnac phase difference between the recombined counterpropagating light beams, the intensity of light due to the interfering beams varies as cos(.phi..sub.s). When the phase difference is close to zero, the cosine function varies only slightly with changes in phase difference. In addition, it is impossible to determine the sign of the phase shift from this operating point. In order to increase the sensitivity of detection, it is advantageous to introduce artificially an added fixed phase shift or "bias" to shift to a point of operation on the cosine curve where the rate of change of intensity with respect to .phi..sub.s is greater. In particular, maximum sensitivity and linearity of response are achieved by introducing a nonreciprocal phase bias such as .pi./2 radians. At this point, the light intensity is proportional to cos(.phi..sub.s +.pi./2 )= sin(.phi..sub.s). The periodic nature of the cosine function results in an equivalent maximum sensitivity and linearity of response (apart from algebraic sign) at any odd integral multiple of plus or minus .pi./2.
Nonreciprocal phase shifts may be induced in a fiberoptic gyro by a reciprocal phase modulator placed near one end of the fiber coil. In order to obviate stability problems, various methods have been proposed for modulating the phase of the light waves propagating within the closed optical path of a Sagnac interferometer.
A phase modulator device can be based, for example, on the change in refractive index with applied voltage in an electro-optic crystal forming part of the closed optical path of the interferometer. If the electro-optic phase modulator is placed near one end of the fiber coil, application of a voltage to the modulator produces a modulation of the phase of one of the counterpropagating waves entering the loop that is not experienced by the other until it has traveled all the way around the coil. The second wave experiences a phase modulation which is delayed by the length of time required for light to propagate around the coil, a time given by EQU Y.sub.o =nL/c,
where n is the index of refraction of the fiber material, L is the length of the fiber coil, and c is the speed of light in vacuum. If V(t) is a time-varying signal applied to the phase modulator, the phase difference between the counterpropagating light waves is proportional to V(t)-V(t-Y.sub.o). In this way a phase bias can be produced which sets the operating point of the interferometer.
If there is a rotation of the fiber coil, a phase shift .phi..sub.s will be added to the phase bias due to the nonreciprocal nature of the Sagnac effect. Although it is possible to use the output signal of the photodetector to estimate the rotation directly, it is preferable to use a "nulling" or "zeroing" method and to estimate the rotation from a feedback modulation signal, in order to avoid errors resulting from drifts in the signal detection electronics. The idea is to generate a feedback modulation signal which introduces a nonreciprocal phase shift in the optical circuit which is equal in magnitude but opposite in sign to the rotationally-induced phase shift, thereby "nulling" or "zeroing" the variation of the intensity signal. Application of the feedback modulation signal to the phase modulator produces a phase difference between the counterpropagating waves which is continuously equal and opposite in sign compared to the phase shift induced by the rotation of the closed optical path. A method such as this in which there is a closed feedback loop is often referred to as a "closed-loop" method.
One method of closed-loop feedback, generally known as the "serrodyne method," makes use of a feedback modulation signal which is a reciprocal phase ramp having a slope proportional to .phi..sub.so /Y.sub.o, where .phi..sub.So is a constant rotationally-induced phase shift and Y.sub.o is the time taken for a light wave to travel around the closed light path of the interferometer in the absence of any rotation. A bias modulation signal consists of a voltage square-wave having an amplitude which induces a phase shift of plus or minus .pi./2 radians and a frequency equal to 1/2Y.sub.o. Since the reciprocal phase ramp signal cannot increase indefinitely, the serrodyne method actually generates a sawtooth feedback waveform with a peak-to-peak amplitude of 2.pi. radians, with the 2.pi. phase transition effectively resetting the operating point of the interferometer to an equivalent position on the intensity interference curve relating output signal to input phase difference.
In a typical serrodyne method a digital phase ramp in the form of a staircase-shaped voltage feedback signal is combined with a bias modulation signal of the type described above. The digital staircase signal consists of a sequence of voltage steps, each of duration Y.sub.o, to the phase modulator. In general, the amplitude of each step change is calculated to provide a nonreciprocal phase shift of plus or minus .pi./2 radians minus a Sagnac phase estimate. The intensity output of the interferometer is demodulated at the bias modulation frequency, namely 1/2Y.sub.o.
The resulting signal is proportional to the residual Sagnac phase shift. It is this signal that a closed-loop controller will act to "null" or "zero." To avoid problems with voltage saturation, the modulation steps are occasionally required to "roll over" in the phase bias resetting operation described above. The step voltage to the phase modulator is adjusted to provide an additional phase shift of plus or minus 2.pi. radians to keep the voltage to the phase modulator in a reasonable operating range. Additional demodulation logic may be employed during these roll-overs to determine the error in estimated phase modulator gain. Through subsequent roll-overs, the estimated phase modulator gain error may be nulled. The phase modulator gain is the proportionality constant relating the phase induced by the phase modulator in response to a given value of input voltage. This secondary loop control, as it is formally known, provides additional scale factor stability to the sensor. The scale factor for a closed-loop rotation sensing interferometer is proportional to the product of the Sagnac scale factor and the phase modulator gain. The Sagnac scale factor is the constant of proportionality between rate of rotation and the Sagnac phase shift.
The cost of electronics is significant in the production of fiberoptic gyro systems. It would be advantageous to be able to use a single set of electronics to control a plurality of fiberoptic gyros.